The present invention relates to a device and a method for control of power flow in a three-phase transmission line, whereby an additional voltage is serially applied to this line, for each of its phases, which additional voltage is generated in dependence on a controllable part of the voltage between the other two phases of the transmission line, and use of such a device for control of the distribution of transmitted power between parallel transmission lines and for damping of oscillations in active power between two power networks interconnected by means of a transmission line.
A transmission line in this context shall mean a three-phase ac line that interconnects two electric power networks and transmits active power between the power networks.
Different kinds of devices for both static and dynamic control of the power flow in such a transmission line are known. The object of the control may be a static distribution of power between power lines or power networks, as well as damping of power oscillations in the transmission line.
One such known device is a so-called phase shifting transformer (PST). The device comprises, for each of the phases of the transmission line, a series transformer, the secondary winding of which is connected into the phase conductor, and a shunt transformer, the primary winding of which is connected between the other two phase conductors. The secondary winding of the shunt transformer is provided with an on-load tap changer and its secondary voltage, which is thus variable, is applied to the primary winding of the series transformer. The additional voltage which arises across the series transformer, and which is thus a series voltage vectorially added to the voltage of the phase conductor, attains, by this connection, a phase position that is displaced by 90xc2x0 relative to the phase voltage of the phase conductor. By varying the amplitude of the additional voltage by means of the on-load tap changer, the power flow in the transmission line is influenced.
Such a phase-shifting transformer will be further described in the following.
As an alternative to the on-load tap changer, the secondary voltage of the shunt transformer may be applied to converter equipment, suitable for the purpose, for electronic control of the amplitude of the secondary voltage, for example by phase-angle control.
The on-load tap changer constitutes a mechanical component that requires maintenance and is subjected to wear. Further, it is relatively slow, the time for a change of the amplitude of the additional voltage being of the order of magnitude of seconds.
Electronic control of the amplitude of the additional voltage may be made faster but, because of its principle of operation, it injects harmonics in the transmission line.
Another such known device is a so-called universal power flow controller (UPFC). A three-phase transformer is connected in shunt connection to the transmission line and the secondary voltage of the transformer is applied to a first three-phase converter of the type pulse-width-modulated, self-commutated voltage-source converter. A second converter of the same kind is connected, by means of a dc voltage intermediate link with a capacitor, to the first converter and the second converter is connected, via its ac terminals, to series transformers connected to the transmission line. As is known, the output voltage of the second converter allows itself to be controlled both with respect to amplitude and phase angle, and may thus be used for a fast and continuous control of both active and reactive power.
The amount of power electronics is, however, relative extensive and complicated and this type of controller is therefore less attractive. Further, it exhibits sensitivity to short-circuit currents and is inclined to apply harmonic and low-frequency harmonics to the transmission line, as well as harmonics associated with the carrier frequency of the pulse-width modulation.
The object of the invention is to provide a device and a method of the kind described in the introduction, which, in relation to the prior art, constitute an improvement with respect to the above-mentioned drawbacks.
According to the invention, this is achieved by coupling, for each of the phases of the transmission line, to the respective phase, a series circuit with a first and a second terminal and a connection point, the series circuit comprising a first reactive impedance element with a fixed reactance connected between the first terminal and the connection point, and a second reactive impedance element with controllable reactance connected between the connection point and the second terminal, whereby one of said terminals is coupled to the respective phase in the transmission line and the other terminal is coupled to a terminal at each of the other two series circuits such that, for all the phases, either the first or the second terminal is coupled to the transmission line, that the additional voltage is formed in dependence on the voltage between the connection points at the other two series circuits, and that the control of the power flow is performed by varying the reactance of the second impedance element.
In an advantageous further development of the invention, the second impedance element comprises a series circuit of one inductive and one capacitive reactance element so dimensioned in relation to each other that the phase position of the additional voltage may be varied to lie both before and after the phase position for the voltage of the transmission line in the respective phase, such that the active power flow in the transmission line may be influenced both in an increasing and a decreasing direction.
In another advantageous further development of the invention, the first impedance element comprises a first fixed inductor and the second impedance element comprises a cross-magnetized inductor with a magnetic core, a main winding for alternating current, and a control winding for direct current, the reactance of the second impedance element being varied by controlling a magnetic flux associated with the main winding by orthogonal magnetization of the magnetic flux in dependence on a direct current applied to the control winding.
In still another advantageous further development of the invention, the first reactive impedance element comprises a first fixed inductor, and the second impedance element comprises inductor equipment with a number of mutually series-connected fixed second inductors, each one of these being parallel-connected to a controllable short-circuit device, the reactance of the second impedance element being varied by respectively activating and deactivating the short-circuit devices.
Additional advantageous further developments of the invention will be clear from the following description and the appended claims.
With a device according to the invention, the following advantages are achieved, inter alia.
Shunt inductors already present in the transmission line may be utilized as a component in the device.
No mechanically movable parts, nor any converter equipment with continuous control are required.
The device does not apply any harmonics to the transmission line.
The device may also be utilized as a shunt inductor to absorb reactive power when control of power flow is of secondary interest.
Since a device according to the invention introduces additional impedance in the power network, both by the series transformer in the transmission line and by the equivalent impedance for the fixed and the controllable inductor, respectively, the short-circuit current in the transmission line is generally reduced.